Measurement of volumetric flow from various production zones into an oil production well is of considerable interest to well and production engineers in order to optimize the oil production and minimize the production of “unnecessary” water.
A lot of the oil on the Norwegian continental shelf, and also elsewhere in the world, is produced by well completions including flow control systems which characteristics depend on the pressure differential between the formation—or the annulus—and the production tubing, such as the so-called ICDs (Inflow Control Devices).
The production or exploration tubing is positioned in the well surrounded by the annulus. A sand screen is placed between the production tubing and the annulus. When ICDs are used, the annulus is isolated in production zones by means of packers between the production tubing and the reservoir. In each production zone, one or more ICDs are placed where the reservoir fluid flows into the production tubing. Possible flow control systems include Inflow Control Devices typically incorporating inlet opening(s), a pressure drop step, and opening(s) towards the pipe. An example is the Halliburton™ Equiflow™, focussed on reducing heel-toe effects or effects of permeability differences. Another example is the ICD described by Statoil in WO 2009/088292, designed to be selective on the fluid allowed in the pipe, and including a valve with an internal self-adjusting movable body. Flow control systems can be integrated to a sliding sleeve device which additional function is upon actuation to open/close/choke production zone or to control communication between the tubing and the annulus. A sliding sleeve device is thus incorporated in the definition of flow control system. Omega Completion Technology Ltd has such a sliding sleeve on sale. Most, if not all, inlet flow control devices have well defined flow characteristics as a function of pressure differential between the annulus and the tubing Q=f(ΔP) such that one may calculate flow if knowing pressure differential and vice-versa.
In any production well, irrespective of the flow control system that is used, it is desirable to measure the production characteristics from each production zone. Various prior art methods exist for measurement of production characteristics.
EP 0816631 discloses the use of radioactive or DNA tracers. These tracers are impregnated on the production zone tubing external coating or packed into small bags glued on perforating charges and inserted in the production formation upon perforation. Different tracers are used for different production zones or regions to enable data for different zones or regions to be isolated and analysed. The tracers are released into the well in quantity increasing with the production flow, but not proportional to the flow, and certain assumptions are required to calculate flow volumes. Release of tracer from a coating will depend on local turbulence and the composition of the fluid flowing from the reservoir, and as only a limited amount of usable tracer can be stored in the coating, operation of this solution will be limited in time before replacement of the pipe is required. Release of tracer from the bags issued from perforation will depend on the homogeneity of their distribution, the rocks characteristics in the close vicinity of the perforation, local turbulences, and fluid composition variations. As a consequence this technique does not provide accurate results.
EP 1277051 discloses a method of monitoring hydrocarbon and water production and detection of phenomena including local variations in pH, salinity, hydrocarbon composition, temperature, pressure and so on. The disclosed technique uses specific tracers immobilised by means of a polymer capable of adhering to the formation. The release of the tracer into the production flow is once again dependent on flow conditions and fluid composition, and will vary in an unpredictable fashion. The lack of proportionality of tracer release and inaccuracy of calculations based on the weakening of ionic or covalent bonds between the tracer and the polymer or between the polymer and the formation is such that accurate flow volume measurements are not possible.
U.S. Pat. No. 6,799,634 describes flow monitoring within a pipe using tracer release as a function of the flow. The tracer reservoir installed beside a venturi is activated by the pressure difference between the inlet and the constriction of the venturi. The tracer is released based on a dynamic pressure differential inside the production tubing, and the volume flow rate inside the production tubing may here be determined, by analysing the quantity of tracer downstream.